Hard and strong abrasive grits, for use in grinding wheels, flexible coated abrasive products ("sandpaper"), or as loose abrasive, are produced commercially from alumina containing raw materials either by fusion in an electric furnace or by the firing of shaped bodies containing finely divided alumina at temperatures well below the fusion points of the material. Such lower temperature process is called sintering. This invention relates to polycrystalline aluminous abrasives and other bodies made by the sintering process.
The first large-scale commercially produced sintered abrasives were produced by the method taught in U.S. Pat. No. 3,079,243 to Ueltz. This patent teaches the milling of calcined bauxite to produce a fine particle size raw material which is then formed into abrasive grit sized particles and fired at about 1500.degree. C. to form hard, strong, tough, pellets of polycrystalline alumina.
Recently, abrasive materials consisting of grits made up of alumina and magnesia spinel, presumably made according to the teachings of U.S. Pat. No. 4,314,827, and made according to the teaching of published British Application 2,099,012A, published Dec. 1, 1982, have been commercially introduced. These materials are produced by the sintering (at about 1400.degree. C.) of dried alumina gel particles. U.S. Pat. No. 3,108,888 to Bugosh also teaches making high density alumina (or alumina containing) products by firing a dried alumina gel made from alpha alumina monohydrate (boehmite) or by hot pressing dried powders made from such gels.
Alumina/magnesia-spinel commercial abrasives made from gels contain alumina in the form of cells from 5 to 15 microns in diameter. The cells, or "sunbursts", are made up of many elongated alumina arms 0.2 to 0.4 micrometers in diameter (but some of which can be as large as 1 micrometer in the form of very roughly spherical "blobs"). The arms in each cell appearing generally to radiate from the center of the cell. All of the arms in a given cell are apparently crystallographically identically oriented. Such orientation is shown by the fact all the area of a given cell extinguishes simultaneously upon sample rotation when viewed between crossed polarizers by transmitted light microscopy.
While the commercial abrasives made from sintered gels containing alumina and magnesia are high quality abrasives, it has not been possible to produce high purity alumina grits by the gel route. This is shown by the relative softness and lack of abrasive utility for the "control" example 13 in U.S. Pat. No. 4,314,827 which was made from an alumina gel without metal oxide or metal salt additions.
The present invention is an improvement in the art of making strong alumina bodies whereby useful abrasive products or other ceramic products can be made from alumina gel with or without the addition of zirconia or spinel formers such as magnesia.